Current state of the art backplanes for active matrix displays, for instance AMOLED displays, use a pixel driver circuit for each light emitting element, for instance each OLED, each pixel driver circuit driving a predetermined current through the corresponding light emitting element. Multiple pixel driver circuit schematics are being implemented, which all comprise a drive transistor driving the predetermined current through the light emitting element. One example is illustrated in FIG. 1, where a light emitting element, an OLED 101 in this case, is coupled in series with a drive transistor M1 between a supply voltage VDD and ground GND. The gate of the drive transistor M1 is connected to a main electrode of a select transistor M2, the gate of which is connected to a select line SA, and the second main electrode of which is connected to a data line DA. A capacitor C1 is coupled between the gate of the drive transistor M1 and the electrode of the OLED 101 coupled to the drive transistor M1.
In an analog driving method an amplitude modulation approach is used, wherein each light emitting element, e.g. OLED, emits light during a full frame period with an intensity corresponding to the required gray level. The current through the light emitting elements, e.g. OLEDs, is determined in accordance with an analog data voltage on the gate of the drive transistor M1. As this transistor M1 preferably operates in saturation for accurate current control, e.g. in order to eliminate or substantially reduce differences in luminance between different light emitting elements, e.g. OLEDs, due to differences in light emitting element, e.g. OLED, threshold voltage, such backplanes are typically driven at power voltages beyond 8 V. The voltage drop over the drive transistor is far higher (typically larger than 4V) than the voltage drop over the light emitting element. This results in more energy being dissipated in the backplane than in the light emitting element. The current through the light emitting element (and thus the light emitting element luminance) varies with the square of the M1 gate voltage. This introduces non-linearities in the display response, limits accuracy and makes the display sensitive to noise.
In a digital driving method a Pulse Width Modulation (PWM) approach can be used, wherein each light emitting element, e.g. OLED, emits light during a portion of a frame period, at a single luminance. In this approach the portion of the frame period during which a light emitting element emits light has a duration corresponding to the required gray level. In an active matrix display, e.g. an AMOLED display, using digital driving based on pulse width modulation, it is preferable to operate the drive transistors in the linear regime to reduce the power consumption of the display. However, when the drive transistor operates in the linear regime there is a variation of electric current through the light emitting elements due to variations in light emitting element characteristics, transistor characteristics or device temperature, and/or due to degradation of the light emitting elements with time. These effects are particularly visible in AMOLED displays. They produce degradation of the image which may lead to, for instance, screen burn-in. Besides, in particular in case of AMOLED colour displays, however not limited thereto, the degradation is uneven in the different colours (blue normally degrades faster than the other colours). Therefore, compensation circuits are typically used for each pixel, resulting in relatively complex pixel driver circuits, with an increased pixel size.
As an alternative to using compensation circuits, methods have been proposed for directly controlling the current through the light emitting elements, e.g. OLEDs, in a digitally driven display. Examples of such driving methods are described in US 2011/0134163. In this approach, each pixel of a display has a current supply circuit, a switch portion and a light emitting element connected in series between a power supply reference line and a power supply line. The switch portion is switched between ON and OFF using a digital video signal. The current supply circuit causes a constant current flowing through the light emitting element (e.g. OLED). Despite that, with this approach, each light emitting element can emit light at a constant luminance even when the current characteristic is changed (for example due to degradation), it is a disadvantage of this solution that the resolution of the display is reduced. The reason is that providing a current supply circuit in each pixel results in a complex pixel circuit with an increased pixel size and thus lower resolution. Also the accuracy of such in-pixel current control may be limited because of transistor matching issues.